Method and system for retrieving an anti-virus signature from one or a plurality of virus-free certificate authorities

ABSTRACT

A method and system as disclosed for use in a virus-free certificate proxy ( 107, 801 ), of retrieving from one or a plurality of virus-free certificate authorities ( 104, 804 ) a virus-free certificate ( 200 ) certifying that a file is virus-free. The method includes the steps of: receiving ( 1001 ) virus-free certificate request for a file; selecting a virus-free certificate authority ( 104, 804 ) having authority to generate a virus-free certificate ( 200 ) for the file; requesting ( 1003 . . . 1007 ) the virus-free certificate to the selected virus-free certificate authority ( 104, 804 ); receiving ( 1003 . . . 1007 ) from the selected virus-free certificate authority the generated virus-free certificate; sending back ( 1005 ) in response to the virus-free certificate request the received virus-free certificate.

This application claim foreign priority benefits under Title 35, United States Code, 119 of European Application No. 00480007.4, filed Jan. 6, 2000, the contents of which are hereby incorporated by reference into this application.

TECHNICAL FIELD

The present invention relates to computer virus and more particularly to a method and system for retrieving an anti-virus signature from one or multiple virus-free certificate authorities.

BACKGROUND ART

Among all computing and networking security issues, the most important cause of concern does not come from intrusions, but from the widespread proliferation of viruses. Viral infections represent the great majority of all security incidents.

Virus Protection

Virus protection for large organizations has become more and more complex and difficult because of:

-   -   the combined use of heterogeneous systems and practices,     -   the widespread use of distributed or client/server systems, and         the free exchange of data files via network sharing, e-mail,         Internet . . . .

Until recently, viral infections threatened only data residing on storage media, such as hard drives and floppy disks. However, with the emergence of macro viruses, the threat has spread to applications. Most organizations are not aware of this level of penetration and are not organized to manage and prevent virus attacks. An effective virus protection software must prevent infections rather than simply treating them after they have already occurred. Anti-virus solutions need a uniform plan, with a centralized control, automated virus signature updates, and support for multiple platforms, protocols, and file types.

Computer Viruses

A computer virus is any program created to reproduce itself. A virus reproduces itself by attaching itself to programs, files, or even to boot sectors of disks. A virus is activated when the infected file or disk is opened or accessed. Once a virus resides in a memory, it can attach itself to the next file or disk accessed, and so on. A virus may be designed to do harm. A virus may also have unintended consequences by overwriting important computer information and by causing costly inconveniences to users and network managers. There are four general types of computer virus:

-   -   File Viruses (including macro viruses), which are attached to         files;     -   Boot sector Viruses in which the boot sectors of floppy or hard         disks are infected;     -   Master Boot Record (MBR) Viruses which infect the disk master         boot record; and     -   Multi-partite Viruses that are a combination of a file virus and         a boot sector virus.         Virus Disguises

Viruses need to avoid detection in order to succeed in corrupting target computers. Simple viruses, with easily detectable signatures are giving way to more sophisticated virus types:

-   -   Polymorphic Viruses: they change their signature, or profile,         each time they are activated so that a fixed signature filter         will miss them.     -   Stealth Viruses: they attempt to hide their presence by         intercepting interrupt services and by feeding back false         information to anti-virus products and end users.     -   Encrypted Viruses: they are delivered within an encrypted file         and are undetectable by a simple anti-virus.         Sources of Infection

Every improvement in network and communication technologies opens new avenues through which viruses can infect your system. Most of former viruses were boot sector viruses, in which the boot sectors of floppy or hard disks were infected.

Macro Viruses

As stated earlier, the creation of macro viruses has changed this environment dramatically. A macro virus is a set of instructions comprising powerful macro routines initially designed for word processing and spreadsheet applications. These macro languages enable a myriad of useful functions which can be imbedded into a document and which can be executed when the document is opened for view or use.

Internet

With the exploding development of the Internet, viruses have catastrophic possibilities. The Internet introduces two different virus threats.

-   -   The first threat is caused by the download of files comprising         viruses when these files are browsed or transferred using for         instance FTP (File Transfer Protocol) routines. Public shareware         (shared software) and executable routines of all types,         including formatted presentations, are a growing source of virus         infection. Furthermore, new Internet virus threats are beginning         to appear in the form of malicious JAVA and Active-X applets.     -   The second threat comes from electronic mail (e-mail). Most         Internet e-mail systems provide a very rich capability to attach         formatted documents to mail sent over the network. These e-mail         messages can be broadcast to individuals or groups of         individuals with the simple stroke of a key! Infected documents         or files can flood a corporate network through gateways and mail         servers. As networking, telecommunications, remote access,         message systems supporting attachments of all kinds become more         and more common, viruses will exploit these new electronic         pathways to attack systems that were heretofore unreachable.         Groupware Complications

A third trend in networking also exacerbates the virus threat: the trend towards the deployment of Groupware applications such as Lotus Notes, Microsoft Exchange, Novell Groupwise, . . . .

Since the active and repeated sharing of documents over the network is at the core of these applications, they represent a fertile ground for the deployment of macro viruses.

A Groupware application not only acts as a repository for shared documents, but, due to its collaborative function, it simultaneously broadcasts files to associated work groups. The broadcast of files significantly multiplies the possibility of accidentally deploying mail infected by attached macro viruses and makes Groupware protection a high priority.

Symptoms of Virus Infection

Most viruses attempt to remain undetected as long as possible to extend their destructive influence. Therefore, most viruses do not produce any recognizable profile or signature that would allow to trap them by scanning the software. However, viruses perform actions that do not look like normal computer operations or user operations. These abnormal actions can be detected by intelligent anti-virus software. Fortunately, many viruses have telltale symptoms and may inadvertently give off signals that can alert users and virus protection software to their presence.

Some of these symptoms include:

-   -   Increase in byte length of files,     -   Alterations of a file's time stamp,     -   Delayed program loading or activation,     -   Reduced performance,     -   Lower system resources, available memory, disk space,     -   Bad sectors on floppies and hard drives,     -   Strange or non-standard error messages,     -   Non-standard screen activity, display fluctuations,     -   Program inoperability (failing to execute),     -   Incomplete or failed system boots, and     -   Uninitiated drive writes.         Anti-virus Software Overview         Detecting a Virus

Viruses are becoming increasingly sophisticated and, as such, can defeat simpler, single dimension software packages. To be effective, the anti-virus software must include special-purpose, distributed applications. Applications can detect viruses using five distinct methods:

-   -   Signature Scanning: This method compares the content of files         against a database of virus signatures. This method requires         frequent updates of the database to ensure the identification of         new and changing signatures.

Integrity Checking: This method compares the profile of current files and disk areas against an archived snap shop of these same items. The detected differences may indicate the presence of a virus. Check summing is the most common type of integrity checking. Unfortunately, integrity checking is generally not effective against modern stealth viruses, so further detecting means are needed.

Heuristic Analysis: An artificial intelligence monitors virus-like behavior, such as trapping certain interrupt services or attempting unlikely actions such as reformatting the hard disk.

Polymorphic Analysis: Polymorphic viruses are difficult to detect because they constantly change their look, particularly when they are encrypted or when they use stealth techniques to hide their presence. A polymorphic analyzer will move any suspect file to a separate, protected, location in the computer and will execute it there to see if it exhibits any virus-like behavior.

Macro Virus Analysis: A specifically designed anti-virus software detects macros in files and tests them before execution.

Archived and Compressed Files

In addition to the support of these five types of virus analysis, an effective anti-virus system must also be able to scan archived and compressed files. Zip (or Pkzip) and Microsoft Compression are the most common tools for archiving and compressing a file. A virus can hide inside a compressed archive, and can remain dormant or unnoticed until the infected file is extracted and released into a system. The minimum for an efficient anti-virus system is to be able to scan most current types of archives to identify viruses stored within the files they contain.

Frequency of Database Signature Update

Finally, the ability of a virus software to prevent virus attacks is determined by its ability to maintain an updated virus signature database. Any anti-virus software must have an associated, easily accessible Web site, or some other online source of information, where regular virus database updates can be retrieved. Products that automate this update process by using an Internet connection to regularly collect new information have a clear advantage in this regard.

Real Time and Scheduled Virus Scanning

Most anti-virus software can perform a scan of a computer in order to detect and possibly treat the viruses found at that time. This process is called scanning. Scanning a computer for viruses can occur:

-   -   at regular intervals under the control of a scheduler, or     -   as an on-demand operation manually executed, or     -   as an event-activated operation (usually in response to some         recognizably “illegal” behavior by a potential virus).

In addition, viruses can be detected in real time, when they are received. This capability is important because if viruses can be detected when they attempt to enter within a system (computer, data repository, server . . . ), then it is possible to prevent them from corrupting other files. Oftentimes, a scheduled scan may occur after a virus has already entered within a computer and has corrupted other files. Obviously, the earlier a virus can be detected, the better.

To be truly useful, an anti-virus software must have the ability to perform all types of scans.

Certificate

A Certificate is a structure that contains a public value (i.e. a public key) associated with an identity. For instance, within a X.509 Certificate, the public key is bound to a “user's name”. A third party (a Certificate Authority) attests that the public key belongs to the user. A X.509 Certificate is a very formal structure and comprises different elements:

-   -   Subject: This is the “user's name” (the Subject can be any         identity value).     -   Issuer: This is the name of the third party that has         issued/generated the certificate. This third party is the         Certificate Authority (CA).     -   Public Key Value: This is the public key of a public/private key         pair. An associated field defines the public key algorithm that         must be used, for instance a RSA, Diffie-Hellman or DSA public         key.     -   Validity: Two fields are used to define the period of validity         (valid from date 1 and valid to date 2).     -   Serial Number: This field provides a unique Certificate serial         number for the issuer.     -   Signature: The signature is an encrypted digest generated by the         Certificate Authority (CA) for authenticating the whole         certificate. The digest results from the hashing of the         Certificate. The digest is encrypted using the CA private key.         The encrypted digest which is the signature, “certifies” that         the Subject is the “owner” of the public and private keys.         Certificate Verification

The Certificate needs to be verified to ensure that it is valid. This is a quite complex process. The verification by an end user of a Certificate comprises the checking of the following elements:

-   -   Valid (or any) Subject and Issuer names are defined in the         Certificate.     -   The Certificate is not expired (checking of the Validity period         field).     -   The Certificate has not been revoked (this may be determined by         obtaining a current Certificate Revocation List from the CA).     -   The signature on the Certificate is valid (the signature is not         verified by using the Certificate's public key but by using the         CA public key).

The method for validating the signature is quite simple, and comprises the steps of:

-   -   extracting the issuer's name (CA name) from the Certificate;     -   locating the issuer's Certificate (CA Certificate) or the         issuer's public key (CA public key).     -   checking that the end user's Certificate signature was generated         by the issuer (CA) using the issuer's public key (CA public         key).

Certificates are generated by a Certificate Authority (CA). Two main methods can be used:

-   -   Centralized Generation: The private/public key pair is generated         by the end user (defined in the subject field of the         Certificate). The public key is directly provided by the end         user to the CA software to create a Certificate. The Certificate         can be provided to another end user via any suitable channel.         The channel does not have to be secure because a Certificate is         a self protecting structure (given the CA's signature).     -   Distributed Generation: The private/public key pair is generated         by the end user. The end user requests the CA to build a         Certificate including the end user public key. The public key is         then sent to the CA for certification. If the request is valid         then the CA returns a Certificate associating the user identity         with the user public key to the end user.

Of course these two methods can be combined in any system, because trusted CA keys are generated by the Certificate Authority (CA).

OBJECTS OF THE INVENTION

Current anti-virus method are becoming more and more complex due to:

-   -   the number of viruses,     -   the difficulty to find them, and the fact that their signature         can change with time or environment.

Virus are coming from everywhere and especially from the Internet network. The time required to check a disk within a computer system, becomes more and more important. Furthermore, the checking of a disk involves the use of resources which may prevent the normal use of the computer system.

It is an object of the present invention to improve current anti-virus checking methods and to provide a new method using file Certificates similar to X.509 Certificates used to authenticate an identity. A specific process associates a Certificate, called virus-free Certificate (VC), with a file in order to speed up and improve the virus detection.

It is another object of the present invention is to reduce the consumption of resources (for instance, the CPU—Central Processing Unit) and to reduce the time necessary to detect viruses within files. This reduction is especially important on systems handling a huge amount of traffic (for instance IP Routers or Firewalls). The performance of such systems is highly impacted by usual anti-virus checkers because usual checkers require to process each file. The detection of viruses on said systems is a very complex process and must be done as fast as possible.

It is another object of the invention, when a system requires a virus-free Certificate for a particular file, to retrieve this virus-Free Certificate as fast as possible, for obvious performance reasons. Copies of existing virus-free Certificates are stored instead of being rebuilt each time they are required. Retrieving an existing Virus-free Certificate is more efficient (saves time and provides better performances) than rebuilding a new virus-free Certificate. Within a network, multiple authorities can be used to build a virus-free Certificate for a particular file. They can share, for instance, the load of building virus-free Certificates and can use different anti-virus checkers. An authority among these multiple authorities must be identified when a virus-free Certificate has to be retrieved.

SUMMARY OF THE INVENTION

The present invention relates to computer viruses and discloses a method and system for retrieving an anti-virus signature from one or multiple systems. The anti-virus signature is comprised in a virus-free Certificate associated with each file. The signature is generated by a trusted server (a Virus-free Certificate Authority—VCA) which avoids the system which receives the file to check this file for all existing virus. The Virus-free Certificate Authority validates the file against all known viruses, using one or several anti-virus checkers. In case of new viruses, only the virus-free Certificates is changed and the only process performed by the system receiving the file (typically a network device) is to verify the file against the signature included in the virus-free Certificate, and to filter the file according to predetermined rules. The present invention drastically simplify the computing resources used for detecting viruses on network devices such as IP Routers and Firewalls. Files on Web Servers can be downloaded with their anti-virus Certificates suppressing the risk of virus. The full anti-virus is processed once on the Virus-free Certificate Authority instead of being processed on each system. Since the processing resources required on the system are limited (because the anti-virus checker is processed on the Virus-free Certificate Authority and not on the network device), the performance impact on the system is also limited.

The present invention relates to computer virus and more particularly to a method and system for use in a virus-free certificate proxy, of retrieving from one or a plurality of virus-free certificate authorities a virus-free certificate certifying that a file is virus-free.

The method comprises the steps of:

-   -   receiving virus-free certificate request for a file;     -   selecting a virus-free certificate authority having authority to         generate a virus-free certificate for the file;     -   requesting the virus-free certificate to the selected virus-free         certificate authority;     -   receiving from the selected virus-free certificate authority the         generated virus-free certificate;     -   sending back in response to the virus-free certificate request         the received virus-free certificate.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel and inventive features believed characteristics of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative detailed embodiment when read in conjunction with the accompanying drawings, wherein:

FIG. 1 describes the different entities involved in the anti-virus system according to the present invention

FIG. 2 describes the content of a virus-free Certificate according to the present invention.

FIG. 3 describes the Virus-free Certificate Rules Table according to the present invention.

FIG. 4 describes the internal logic of a Virus-free Certificate Firewall according to the present invention.

FIG. 5 describes the Virus-free Certificate Cache Table according to the present invention.

FIG. 6 describes the internal logic of the Virus-free Certificate Cache for retrieving a Virus-free Certificate according to the present invention.

FIG. 7 describes the internal logic of the Virus-free Certificate Cache for updating a Virus-free Certificate according to the present invention.

FIG. 8 describes the different anti-virus entities in a Virus-free Certificate Proxy environment according to the present invention

FIG. 9 describes the Virus-free Certificate Proxy Table according to the present invention

FIGS. 10 a and 10 b describe the internal logic of the Virus-free Certificate Proxy according to the present invention

PREFERRED EMBODIMENT OF THE INVENTION

Introduction

FIG. 1 describes the different entities involved in the virus system disclosed in the present invention. In most of the cases, the file that the Client Workstation (101) requires is stored in a File Server (for instance a WEB Server) (103). A Certificate is associated with this file on the File Server (103). The Certificate indicates that the file has been processed by a list of anti-virus programs (also referred to as anti-virus checkers), and is virus free. It is an object of the Virus-free Certificate Authority (VCA) (104) to build such Certificates (called Virus-free Certificates—VC). The Client Workstation, the File Server, the Virus-free Certificate Authority are attached to a LAN/WAN network (102) (Local Area Network/Wide Area Network). which can include the Internet network.

The Client workstation (101) downloads the file and the associated anti-virus Certificate from the File Server (103) through a network device within the WAN/LAN network. Said network device is for instance:

-   -   an IP (Internet Protocol) Router which routes files between File         Servers and Client Workstations,     -   a Firewall protecting the secure side of the LAN/WAN network         (typically the Client Workstations attached to an Intranet         network) from the unsecured side of the LAN/WAN network         (typically the File Servers attached to the Internet network).

The network device controls and filters each file it receives, according to some anti-virus criteria. For that, it uses:

-   -   predefined rules, and     -   the virus-free Certificate associated with each file.

The network device therefore protects the Client Workstations from viruses. This network device is called Virus-free Certificate Firewall (also referred to as VCF) (105).

Virus-free Certificate Firewall

Depending on some predefined rules, a Virus-free Certificate Firewall (VCF) may have to add a Virus-free Certificate to a file it receives from a File Server. In this case, the Virus-free Certificate Firewall retrieves the virus-free Certificate from a Virus-free Certificate Cache (VCC) (106). If the virus-free Certificate cannot be retrieved from a Virus-free Certificate Cache, the Virus-free Certificate Firewall retrieves the Virus-free Certificate from a Virus-free Certificate Proxy (107).

Virus-free Certificate Cache

A Virus-free Certificate Cache (VCC) (106) is used to store existing Virus-free Certificates within the LAN/WAN network. These Virus-free Certificates can be retrieved by systems attached to the LAN/WAN network (typically the Virus-free Certificate Firewall). Retrieving an existing Virus-free Certificate is a more efficient operation (in term of performance) than building a new Virus-free Certificate.

Multiple Virus-free Certificate Caches can be attached to the LAN/WAN network. In this case, the multiple Virus-free Certificate Caches communicate across the LAN/WAN network in order to exchange the Virus-free Certificates.

Virus-free Certificate Proxy

A Virus-free Certificate Proxy (VCP) (107) acts as a single Virus-free Certificate Authority (VCA) within the LAN/WAN network in order to provide virus-free Certificates to any system attached to the LAN/WAN network. When multiple Virus-free Certificate Authorities (VCAS) are available (for instance, one VCA per software vendor), the Virus-free Certificate Proxy (VCP) identifies a Virus-free Certificate Authority (VCA) which has authority for building a Virus-free Certificate for of a particular file, and retrieves the Virus-free Certificate from said identified Virus-free Certificate Authority (VCA).

Virus-free Certificate Retrieval

Any system attached to the LAN/WAN network can retrieve a virus-free Certificate (VC) for a specific file from either:

-   -   A Virus-free Certificate Authority (VCA), which builds the VC.         The virus-free Certificate (VC) is said to be “authoritative”         because the Virus-free Certificate Authority (VCA) has authority         to build a valid VC, using for instance the latest anti-virus         programs and levels.     -   A Virus-free Certificate Proxy (VCP), which relays the request         for virus-free Certificate (VC) to one or multiple Virus-free         Certificate Authorities (VCAs). Since a Virus-free Certificate         Proxy (VCP) dynamically retrieves “authoritative” virus-free         Certificates (VCs) from VCAs, it also provides “authoritative”         virus-free Certificates (VCs).     -   A Virus-free Certificate Cache (VCC). A VCC has not authority to         build virus-free Certificates (VCs) but it maintains local         copies of virus-free Certificates (VCs). Possibly, a virus-free         Certificate (VC) retrieved from a Virus-free Certificate Cache         (VCC) may be invalid. For instance, the virus-free Certificate         (VC) is expired since it was initially built by a Virus-free         Certificate Authority (VCA). A Virus-free Certificate Cache         (VCC) does not provide “authoritative” virus-free Certificates         (VCs).         Virus-free Certificate

FIG. 2 describes the content of a virus-free Certificate (VC) according to the present invention. The virus-free Certificate reuses the standard X.509 Certificate format. It comprises the signature of the file and therefore is bound to this file. The main difference between a X.509 Certificate and the virus-free Certificate is that the virus- free Certificate comprises:

-   -   an anti-virus name and level;     -   a signature of the file.

The virus-free Certificate (200) includes the following fields:

-   -   File name (201): This is the “name” of the file protected that         the virus-free Certificate protects.     -   Issuer (202): This is the “name” of the third party that         issued/generated the virus-free Certificate. This third party is         the Certificate Authority (CA).     -   Public Key Value (203): This is the public key of a         public/private key pair. An associated field defines the public         key algorithm that must be used to check the file signature, for         instance a RSA, Diffie-Hellman or DSA public key. The public key         is provided by the Virus-free Certificate Authority. The         corresponding private key is used by the VCA to build the         signature of files. So the same private/public key pair may be         used to build several virus-free Certificates from the same         issuer (VCA). This public key within the virus-free Certificate         is preferably used instead of the Virus-free Certificate         Authority public key used to validate only the present         certificate signature and not the file signature. A public key         for decrypting the imbedded signature is added within the         virus-free Certificate because the Virus-free Certificate         Authority public key is generally longer and more complex. The         validity of keys may also differ between the Virus-free         Certificate Authority public key and the virus-free Certificate         public key. Anyway, because the virus-free Certificate is signed         by the Virus-free Certificate Authority, the use of the         virus-free Certificate public key is secure.     -   Validity (204): Two fields are used to define the period of         validity (valid from date 1 and valid to date 2).     -   Serial Number (205): This field provides a unique virus-free         Certificate serial number for the issuer.     -   Certificate Signature (206): The certificate signature is an         encrypted digest generated by the Virus-free Certificate         Authority (VCA) for authenticating the whole Certificate. The         digest results from the hashing of the virus-free Certificate.         The digest is encrypted using the Virus-free Certificate         Authority (VCA) private key. The certificate signature results         from the encrypted digest and “certifies” that the file         signature is encrypted by the private key associated with the         virus-free certificate public key (203). The Virus-free         Certificate Authority (VCA) public key is different from the         virus-free Certificate public key and is either preloaded in the         web browser or given by a trusted entity. The VCA public key is         used to retrieve the original hashing of the full certificate.         The Virus-free Certificate Authority (VCA) can use the same set         of virus-free certificate private/public keys (203) for all the         files generated during a given period of time so the         cross-checking of the issuer authentication can be easily         performed time to time, when a new set of keys is used. Once the         virus-free Certificate public key for a issuer is validated, it         can be reused for several files certified by the same issuer         which reduces the number of virus-free Certificate public keys.     -   File Signature (207): The File Signature is verified using the         public key value given in the virus-free Certificate. The file         signature (hashing) performed by the Virus-free Certificate         Authority (VCA) on the file is encrypted using the VCA private         key. Using the VCA public key allows to retrieve the original         file signature.     -   Anti-virus Checker (208): This field gives an indication of how         the virus-free Certificate has verified that the file was         virus-free. The Anti-virus Checker comprises the name and the         level of the anti-virus program. Several anti-virus programs and         levels may be appended to reinforce the efficiency of the         anti-virus detection.     -   Certificate Structure (209): This field describes the size and         the content of the virus-free Certificate fields. The number or         anti-virus program is defined in this field.     -   If the virus-free Certificate uses a standard format (minimum         size of a virus-free Certificate), this field is optional.     -   If the size of the virus-free Certificate is above the size of         the standard format (above the minimum size), this field is         mandatory and defines the size of the fields comprised in the         virus-free Certificate.         Virus-free Certificate Rules Table

FIG. 3 describes the table used by the Virus-free Certificate Firewall (VCF) (105). Said table provides some indications for controlling and filtering the files received by the Virus-free Certificate Firewall (VCF). This table is called Virus-free Certificate (VC) Rules Table (301).

The Virus-free Certificate Table (301) (a flat file in a preferred embodiment) is typically created by the Network Administrator in charge of the LAN/WAN network. This table associates with each file some anti-virus criteria. Said anti-virus criteria are used by the Virus-free Certificate Firewall (VCF) for controlling and filtering of each file.

Each file is identified by its characteristics (303), which can be one or a combination of the following information:

-   -   the source system which has originated the file,     -   the file name,     -   the file type,     -   optionally, the file size,     -   optionally, the file CRC (a file signature).

The anti-virus criteria (307) associated with each file comprise:

-   -   an information indicating whether or not a Virus-free         Certificate is required for this file,     -   an information indicating a list of information comprised within         a virus-free Certificate (VC) and that must be check to         determine that the virus-free Certificate (VC) is valid,     -   an information indicating whether or not the file must be         discarded, when a virus-free Certificate (VC) is required for         this file, and when either:     -   no Virus-free Certificate is associated with the file, or     -   the Virus-free Certificate which is associated with the file, is         not valid.     -   a list of anti-virus programs and levels that must be used for         building the Virus-free Certificate associated with the received         file. Typically, this list of anti-virus programs and levels is         used by the authority (a Virus-free Certificate Authority) which         builds the virus-free Certificate (VC).

The Virus-free Certificate Rules Table (301) comprises a list of records (302). In order to minimize the number of records in the table, files having the same characteristics are grouped within file templates (303). The anti-virus criteria comprised within one record, apply to all files within the file template. There is one record for each file template, each record comprising the following information:

-   -   (303) a File Template section, which comprises the following         information (fields in the record):     -   (304) File_(—)Source: This is the identifier of the source         system or the plurality of source systems that have originated         files. For instance, the File_(—)Source can be the IP address of         the File Server which has originated all files within a specific         software company (for instance the IP address of www.ibm.com         File Server).     -   (305) File_(—)Name. This is the name of one or multiple files.         The File_(—)Name can be either:     -   an explicit file name which identifies a unique file. For         instance a File_(—)Name with the value “setup1” identifies the         file called “setup1”.     -   a wildcarded file name which identifies multiple files. For         instance, File_(—)Name with the value “setup*” identifies any         file which name starts with “setup” (for instance “setup1”,         “setup_(—)ok”, . . . ).     -   (306) File_(—)Type. This is the type of the file (for instance         “exe”, “doc”, “avi”, “html”, . . . ). A File_(—)Type identifies         any file of this type. For instance, a File_(—)Type with the         value “exe” identifies all files which type is “exe” (for         instance “start.exe”, “word.exe”, . . . ).

Optionally, the File Template section can also comprise a file size. The file size is then the size of all files within the same File Template. The file size can possibly used to differentiate files having the same File_(—)Source, File_(—)Name, and File_(—)Type.

Optionally, the File Template section can also comprise a file CRC (a file signature). The file CRC is then the signature of the file identified by the File Template. The file CRC can possibly used to differentiate files having the same File_(—)Source, File_(—)Name, and File_(—)Type.

-   -   (307) a File Anti-virus Criteria section, which comprises the         following information (fields in the record):     -   (308) VC_(—)Required. This information indicates whether or not         a Virus-free Certificate must be associated with any file within         the file template. The possible values of VC_(—)Required are         “yes” and “no”.     -   Valid_(—)VC_(—)Required. This information indicates the list of         information comprised within a virus-free Certificate (VC) and         which must be validated by the Virus-free Certificate Firewall         (VCF) in order to determine that the virus-free Certificate (VC)         is valid. For instance, a virus-free Certificate (VC) with an         expired date may be considered as valid. This information         applies to any file within the file template. By default, all         information comprised within a virus-free Certificate (VC) must         be checked and validated by the Virus-free Certificate Firewall         (VCF) in order to determine that the virus- free Certificate         (VC) is valid.     -   (310) File_(—)Discard. This information indicates whether or not         the file must be discarded, when “VC_(—)Required” is set to         “yes”, and when either:     -   no Virus-free Certificate is associated with the file, or     -   the Virus-free Certificate which is associated with the file, is         not valid. The possible values are “yes” and “no”.     -   (311) Required_(—)AV_(—)Checker_(—)List. This is the list of         anti-virus programs and levels which must be used for the         anti-virus processing of any file within the file template. Said         list is therefore the list of anti-virus programs and levels         that must be used by the authority (typically a VCA) which         builds the Virus-free Certificate associated with any file         within the file template.

A default record is defined in the Virus-Free Certificate Rules Table. This default record comprises anti-virus criteria that must be used by the Virus-free Certificate Firewall (VCF) for processing files which are not explicitly listed in a specific record.

Virus-free Certificate Firewall

According to the present invention, a purpose of a Virus-free Certificate Firewall (VCF) is to control and filter each file it receives, according to some anti-virus criteria and using Virus-free Certificates. For each file it receives, the Virus-free Certificate Firewall (VCF) performs the following operations:

-   -   retrieving the anti-virus criteria for said received file, from         the Virus-free Certificate Rules Table (301),     -   checking whether or not said received file is associated with a         Virus-free Certificate,     -   checking whether or not the Virus-free Certificate which is         associated with said received file, is valid, based on said         anti-virus criteria,     -   when required by said anti-virus criteria, retrieving a valid         Virus-free Certificate associated with said file. Typically,         said Virus-free Certificate is retrieved either from a         Virus-free Certificate Authority (VCA), or from Virus-free         Certificate Cache (VCC) or from a Virus-free Certificate Proxy         (VCP),     -   discarding the file, when the anti-virus criteria are not         satisfied.     -   forwarding the file, possibly along with its associated valid         Virus-free Certificate, when the anti-virus criteria are         satisfied.

Typically, the Virus-free Certificate Firewall (VCF) is either a dedicated network device attached to the LAN/WAN network, or an existing network device (for instance an existing Firewall) which is enhanced to provide the Virus-free Certificate Firewall (VCF) functions.

FIG. 4 is a flow chart which refers to the internal logic of the Virus-free Certificate Firewall (VCF). The VCF:

-   -   (401): receives a file     -   (402): retrieves the anti-virus criteria associated with the         file, from the Virus-free Certificate Rules Table (403). Said         file anti-virus criteria are comprised in the VC Rules Table         record which is associated with the received file. Said record         is identified using its “File_(—)Source”, “File_(—)Name”, and         “File_(—)Type” fields, and using information retrieved from the         received file (its name, its type, and its source which is         typically identified by the IP address of the system which         originated the file). Typically, the anti-virus criteria are         comprised in the fields (“VC_(—)Required”,         “Valid_(—)VC_(—)Required”, “File_(—)Discard”, and         “Required_(—)AV_(—)Checker_(—)List”) of the file anti-virus         criteria section of the identified record.     -   (404): tests whether or not a Virus-free Certificate (VC) is         associated with the received file. Preferably, the virus-free         Certificate (VC) has been received within the file or through         separate means.     -   If a virus-free Certificate (VC) is found:     -   (405) tests whether or not the received virus-free Certificate         (VC) is valid. Typically, and by default, the VC is valid when         all the following conditions are satisfied:     -   The VCA which has issued the VC, and identified by the “issuer”         field of the VC, is a trusted VCA. Typically, this is done by         checking that the VCA is comprised within a list of trusted VCAs         This list is configured on the VCF by a Network Administrator.

The VC is authenticated using:

-   -   its “certificate signature” field and     -   the public key of the VCA which has issued the VC.         -   Typically, the VCF retrieves said public key from a list of             trusted VCA certificates which have been placed on the VCF             by a Network Administrator, or which have been retrieved by             the VCF through secure means.     -   The VC date is correct. Typically, the current date is comprised         in the date interval provided in the “validity” field.     -   The VC has not been revoked. Typically, this is done using:     -   the “issuer” and the “serial number” fields of the VC, and     -   a certificate revocation list (CRL) issued by the VCA.

Possibly, the “Valid_(—)VC_(—)Required” indication retrieved from the VC Rules Tables in step (402), indicates the list of the above conditions that must be satisfied. For instance, “Valid_(—)VC_(—)Required” may indicate that a VC with an expired date is considered as valid.

-   -   If the virus-free Certificate (VC) is valid:     -   (406) tests whether or not the anti-virus processing of the file         is OK. This anti-virus processing consists in checking that the         VCA which issued the VC has tested the file for viruses. This         checking uses at least the list of required anti-virus programs         and levels comprised in the file anti-virus criteria.     -   The list (List1) of anti-virus programs and levels used by the         VCA to test the file for viruses is retrieved from the         “anti-virus checker” fields of the VC.     -   The list (List2) of anti-virus programs and levels that must be         used to test the file for viruses, is retrieved from the         “Required_(—)AV_(—)Checker_(—)List” field of the file anti-virus         criteria section of the selected record (said selected record         has been retrieved in (402) from the VC Rules table).

By default, the anti-virus processing is OK when List2 is a subset of List 1.

Possibly, the “Valid_(—)VC_(—)Required” indication which has been retrieved from the VC

Rules Tables in step (402), indicates if the anti-virus processing is OK when:

-   -   Both List1 and List2 are identical, or     -   List2 is a subset of List1 (this is the default).     -   If the anti-virus processing is OK:     -   (407) tests whether or not the file signature is OK. This test         compares the file signature calculated by the VCF and the file         signature comprised within the VC (in the “file signature”         field). If both are the same, then the file signature is OK,         which means that the received file has not been modified since         it was checked for viruses by the VCA.     -   If the file signature is OK:     -   (408) updates a Virus-free Certificate (VC) Cache. An update         request comprising the file VC, is sent to a VC Cache by the         VCF. Said request typically comprises the file name and type,         the file size, the file CRC, and the file Virus-free         Certificate. Preferably, the address of a VC Cache is a         predefined parameter of the VCF.     -   This step is optional, and is obviously only done when a VC         Cache has been defined within the LAN/WAN network.     -   (409) forwards the received file, along with its valid VC.

The VCF then waits for the next file to process in (401).

If the file signature is not OK:

-   -   (410) stores an information indicating that the file signature         is not OK (or any other detected error).     -   (411) retrieves a VC from a Virus-free Certificate (VC) Proxy.         The VCF sends a request to a VC Proxy, in order to retrieve a VC         associated with the received file. Said VC is authoritative         since it is built and issued by a VCA. Possibly, the VCF         retrieves an authoritative VC directly from a VCA, when no VC         Proxy is defined within the LAN/WAN network. Said request         typically comprises:     -   The file name and type, the file size (optional), and the file         CRC.

Optionally, a reduced VC which contains the “issuer” information. Said “issuer” information identifies the preferred VCA, and provides the VCP with some selection criteria which can be used when multiple VCs are available for the same file (typically, one VC per VCA).

-   -   Optionally, a list of anti-virus checkers, that should         preferably be used by the VCA to build the VC.

Preferably, the address of the VC Proxy (or VCA) is a predefined parameter of the VCF.

-   -   (412) tests whether or not a valid VC has been retrieved.

A VC may have been retrieved from the VCP (or VCA) in step 411. Said VC is tested to make sure it is valid and has not been corrupted for instance by a malicious system stealing the identity of a trusted VCA. The VC is validated using the same criteria and possibly the “Valid_(—)VC_(—)Required” indication, as in steps (405), (406), and (407).

Typically, the test determines that:

-   -   The VCA which has issued the VC is a trusted VCA.     -   The VC is authenticated.     -   The VC date is correct.     -   The VC has not been revoked.     -   The anti-virus processing is OK.     -   The file signature is OK.     -   If a valid VC has been retrieved:     -   (408) updates a Virus-free Certificate (VC) Cache.     -   (409) forwards the received file, along with its valid VC. The         VCF then waits for the next file to process in (401).     -   If a valid VC has not been retrieved:     -   (413) discards the file, and stores an information which         indicates the discard and that a valid VC has not been         retrieved. The VCF then waits for the next file to process in         (401).     -   If the anti-virus processing is not OK:     -   (414) tests whether or not the VC has been retrieved from a VC         Cache. The information indicating that the VC has been retrieved         from a VC Cache is set in step (420).     -   If the VC has been retrieved from a VC Cache (in step (420))     -   Continues in step (410), in order to possibly retrieve an         authoritative VC from a VC Proxy (or VCA).     -   If the VC has not been retrieved from a VC Cache     -   Continues in step (416), in order to possibly retrieve a VC from         a VC Cache.     -   If the VC is not valid:     -   Continues in step (414), in order to possibly retrieve a valid         VC, either from a VC Proxy (or VCA) or from a VC Cache.     -   If a VC is not found:     -   (415) tests whether or not a VC is required for the received         file. The test uses the “VC_(—)Required” indication retrieved         from the VC Rules Table in step (402).     -   If a VC is required for the received file:     -   (416) stores an information indicating an error condition.         Possibly, a file which should have a VC has been received         without a VC, or a file has been received with an invalid VC.     -   (417) tests whether or not the file must be discarded. The test         uses the “File_(—)Discard” indication retrieved from the VC         Rules Table in step (402). The file must be discarded when         “File_(—)Discard” is set to “yes”, and when “VC Required” is set         to “yes”, and when either:     -   no VC is associated with the file, or     -   the VC which is associated with the file, is not valid.     -   If the file must be discarded:     -   (418) discards the file, and stores an information which         indicates the discard. The VCF then waits for the next file to         process in (401).     -   If the file must not be discarded:     -   (420) retrieves a VC from a Virus-free Certificate (VC) Cache.         The VCF sends a request to a VC Cache, in order to retrieve a VC         associated with the received file. Said request typically         comprises:     -   The file name and type, the file size (optional), and the file         CRC.     -   Optionally, a reduced VC which contains the “issuer”         information. Said “issuer” information identifies the preferred         VCA, and provides the VCC with some selection criteria which can         be used when multiple VCs are available for the same file         (typically, one VC pr VCA).     -   Optionally, a list of anti-virus checkers. Said list indicates         the list of anti-virus checkers that should preferably be used         to build the VC, and provides the VCC with some selection         criteria which can be used when multiple VCs are available for         the same file (typically, one VC per VCA).

Preferably, the address of the VC Cache is a predefined parameter of the VCF. This step is optional, and is obviously only done when a VC Cache has been defined within the VCF.

-   -   (421) tests whether or not a VC has been retrieved from a VC         Cache:     -   If a VC has been successfully retrieved from a VC Cache:     -   Continues in step (405) in order to validate the retrieved VC.         Since the VC has been retrieved from a VC Cache, the VC is not         authoritative and must be validated. An indication that the VC         has been retrieved from a VC Cache is also stored for internal         use (in step (414)).

If a VC has not been successfully retrieved from a VC Cache:

-   -   Continues in step (411) in order to retrieve an authoritative VC         from a VC Proxy (or from a VCA).         Virus-free Certificate Cache Table

FIG. 5 describes the table used by the Virus-free Certificate Cache (VCC) (106). This table, called Virus-free Certificate Cache Table, is dynamically built by the VCC and comprises a local copy of Virus-free Certificates which have been transmitted through the LAN/WAN network. The table (501) comprises for each file, one or multiple associated Virus-free Certificates.

In the VC Cache Table (501), each file is identified by its characteristics (503), which are:

-   -   the file name,     -   the file type,     -   optionally, the file size,     -   the file CRC.

In the VC Cache Table, each file is associated with one or multiple VC sections (508), each VC section comprising the following information:

-   -   The VC associated with this file,     -   The date of the latest request to retrieve said VC,     -   The number of requests (hits) to retrieve said VC.

The table comprises a list of records (502). There is one record for each file, each record comprising the following information:

-   -   (503) a File section comprising the following information         (fields in the record):     -   (504) File_(—)Name. This is the name of the file. Typically,         this is the explicit file name. For instance a File_(—)Name with         the value “setup1” identifies the file called “setup1”.     -   (505) File_(—)Type. This is the type of the file (for instance         “exe”, “doc”, “avi”, “html”, . . . ).     -   (506) File_(—)Size. This is the size of the file. This field is         optional.     -   (507) File_(—)CRC. This is the CRC (a file signature) of the         file.

The combination of File_(—)Name and File_(—)Type may identify in a unique way one file. If multiple files have the same name and type (for instance, multiple “setup.exe” files originated from multiple sources), each one of these said file may then be differentiated in a unique way by its file size (for instance the size of one “setup.exe” is 2 kilobytes, and the size of another “setup.exe” is 4 kilobytes).

If multiple files have the same name, type, and size, each one of these said file is then differentiated in a unique way by its file CRC which is a file signature (hashing)

-   -   (508) a Virus-free Certificate (VC) section comprising the         following information (fields in the record):     -   (509) Virus-free_(—)Certificate (VC). This is the VC associated         with the file.     -   (510) Last_(—)Date. This is the date of the latest request         received by the VC Cache to retrieve or to update this VC.         Typically, this date is used when the VC Cache is maintained and         when for instance the oldest records have to be deleted.     -   (511) Number_(—)Hits. This is the number of requests (hits) that         have been received by the VC Cache to retrieve this VC.         Typically, this number of hits is used when the VC Cache is         maintained and when for instance the records with the lowest         number of hits have to be deleted.

Possibly, multiple VC sections are associated with one file within the same record to support multiple VCs per file. This is for instance necessary when one file has one VC per VC Authority (VCA), each VC Authority using for instance one specific anti-virus program. In this case, each record is still identified by its File section, but multiple VC sections are then associated with said File section (one VC section per VC).

Virus-free Certificate Cache

According to the present invention, a Virus-free Certificate Cache (VCC) (106) stores existing Virus-free Certificates within the LAN/WAN network. Said existing Virus-free Certificates can then be retrieved by systems attached to the LAN/VAN network (typically the Virus-free Certificate Firewall). Retrieving an existing Virus-free Certificate is more efficient than building a new Virus-free Certificate, and therefore provides better performance.

Multiple Virus-free Certificate Caches can be attached to the LAN/WAN network. In this case, the multiple VCCs communicate across the LAN/WAN network in order to exchange the Virus-free Certificates.

The VC Cache performs the following functions:

-   -   Storing copies for each file of the Virus-free Certificate (VC)         or of the multiple Virus-free Certificates in a VC Cache Table.     -   Processing each request for updating the VC Cache Table with a         new Virus-free Certificate. This operation comprises the steps         of:     -   Dynamically updating a local VC Cache Table with a copy of said         new Virus-free Certificate.     -   Possibly updating one or multiple remote VC Cache Tables with         said new Virus-free Certificate.     -   Processing each request for retrieving a VC associated with a         specific file. This operation comprises the steps of:     -   Preferably retrieving a VC from a local VC Cache Table.

Possibly retrieving a VC from a remote VC Cache Table.

Maintaining the VC Cache Table, by discarding from the VC Cache Table the invalid VCs.

Possibly pre-loading the VC Cache Table with one or multiple VCs which are directly retrieved from one or multiple VC Authorities or VC Proxies.

Furthermore, the operation of preferably retrieving a VC from a local VC Cache Table, comprises the following steps:

-   -   Identifying a preferred VC in the VC Cache Table, using one or         multiple selection criteria, said selection criteria comprising:     -   the name of said specific file,     -   the type of said specific file,     -   optionally the size of said specific file,     -   the CRC of said specific file,     -   optionally a preferred list of anti-virus programs and levels,     -   optionally the identifier of a preferred VCA.     -   Retrieving said preferred VC from the VC Cache Table.

Typically, the VCC is either a dedicated network device attached to the LAN/WAN network, or an existing network device (for instance an existing WEB Proxy system) which is enhanced to provide the VCC functions.

Virus-free Certificate Cache Table Maintenance

The VCC periodically maintains the VC Cache Table. This operation comprises the following steps:

-   -   Checking the validity of each VC in the VC Cache Table using the         VC fields. Typically, this checking uses the “validity” and the         “serial number” fields of the VC to determine if the VC has         expired or if the VC has been revoked.

Discarding from the local VC Cache Table:

-   -   each VC section with an invalid VC,     -   Each record with no VC section.

The VC Cache Table maintenance is preferably performed at regular time intervals, for instance every night. The maintenance process can be triggered either automatically or manually. The periodicity of said maintenance is for instance a predefined parameter of the VCC.

Optionally, the VC Cache can discard in one or multiple remote VC Cache Tables, records comprising a VC which is no longer valid. This operation is optional since preferably each VCC maintains its own local VC Cache Table, in order to avoid unnecessary traffic across the LAN/WAN network.

Virus-free Certificate Cache Table Pre-Loading

The VC Cache can pre-loads its local VC Cache Table with one or multiple VCs directly retrieved from one or multiple VC Authorities (VCAs) or VC Proxies (VCPs). In this case, the VC Cache is configured (typically by a Network Administrator) with a list of VCAs (or VCPs). The VC Cache periodically retrieves VCs from the VCAs within said list, and populates the VC Cache Table with records comprising said retrieved VCs.

Typically, the VC Cache can retrieve from a VCA one file (for instance using FTP) comprising the latest VCs created by said VCA. The VC Cache then creates one record in the VC Cache Table, for each VC comprised within said retrieved file.

The pre-loading of the VC Cache Table is preferably performed at regular time intervals, for instance every night. The pre-loading process can be triggered either automatically or manually. The periodicity of said pre-loading is for instance a predefined parameter of the VCC.

Virus-free Certificate Cache: Virus Free Certificate Retrieval

FIG. 6 is a flow chart which refers to the internal logic of the Virus-free Certificate Cache (VCC) and more particularly to the method for processing a request for retrieving a VC for a particular file. The Virus-free Certificate Cache (VCC):

-   -   (601): receives a request to retrieve a VC for a particular         file. Typically, said request for VC typically comprises:     -   The name, and type of the file,     -   The size of the file (optionally),     -   The CRC of the file. This is a file signature identifying the         file.

Optionally, the list of anti-virus programs and levels (referred to as “RCV_(—)AV_(—)Checker_(—)List”), that should preferably be comprised within the list of anti-virus programs and levels which have been used to build the requested VC. Typically, this information is used by the VCC to select the preferred VC, when multiple VCs are available for the same file.

-   -   Possibly, the list is empty, which means that any anti-virus         program and level can be used.     -   Optionally, a reduced VC comprising only the “issuer” field         (referred to as “RCV_(—)issuer”), and identifying the preferred         VCA. The reduced VC then provides the VCC with some selection         criteria. These selection criteria can be used when multiple VCs         are available for the same file (typically, one VC available per         VCA).     -   (602) retrieves one VC from the VC Cache Table (603).

Typically, this operation comprises the following steps:

-   -   Selecting one record (called “Record_(—)S”) within the VC Cache         Table, using:     -   Its File_(—)Name and File_(—)Type, which must be identical to         the file name and file type comprised in the received request.

If multiple records (files) have the same File_(—)Name and File_(—)Type (for instance, multiple “setup.exe” files originated from multiple sources), each record (file) can then be differentiated in a unique way by using the File_(—)Size (for instance the size of one “setup.exe” is 2 kilobytes, and the size of another “setup.exe” is 4 kilobytes).

-   -   In the unlikely event of multiple records (files) having the         same File_(—)Name, File_(—)Type, and File_(—)Size, each record         (file) is then differentiated in a unique way by using the         File_(—)CRC.

Selecting one VC within said selected record:

-   -   When only one VC section is comprised within “Record_(—)S”, the         selected VC is the VC comprised within said VC section.

When multiple VC sections are comprised within “Record_(—)S”, each one comprising one VC, the selected VC is preferably identified using:

-   -   The “issuer” field of the VC. This issuer field is equal (or         comprises) the “RCV_(—)issuer” possibly received in the request.     -   The “Anti-virus Checker” fields of the VC. These Anti-virus         Checkers are equal (or comprise) the         “RCV_(—)AV_(—)Checker_(—)List” possibly received in the request.     -   (604) tests whether or not a VC has successfully been retrieved         from the VC Cache Table.     -   If a VC has been retrieved from the VC Cache Table     -   (605) updates in the VC Cache Table (603), the “Last_(—)Date”         and “Number_(—)Hits” fields of the VC section comprising the VC.     -   Updates “Last_(—)Date” with the current date     -   Increments “Number_(—)Hits”.     -   (606) answers the received request for VC, with a positive         response comprising the retrieved VC. Optionally, the VC is         validated before being sent back in the response. The VCC then         waits for the next request for VC.     -   If a VC has not been retrieved from the VC Cache Table     -   (607) retrieves the VC from a remote VC Cache. The VCC forwards         the received request to one or multiple VC Caches. The address         of said VC Caches is typically a predefined parameter of the         VCC.     -   (608) tests whether or not a VC has been retrieved from a remote         VC Cache.

If a VC has successfully been retrieved from a remote VC Cache:

-   -   (609) stores said retrieved VC in the VC Cache Table. Either a         new record is created in the VC Cache Table, or a new VC section         is created in an existing record.     -   (606) answers the received request for VC, with a positive         response comprising the VC which has been retrieved. Optionally,         the VC is validated before being sent back in the response. The         VCC then waits for the next request for VC.     -   If a VC has not been retrieved from a remote VC Cache:     -   (610) answers the received request for VC, with a negative         response since no VC has been retrieved. The VCC then waits for         the next request for VC.         Virus-free Certificate Cache Table Update

FIG. 7 is a flow chart which refers to the internal logic of the Virus-free Certificate Cache (VCC) and more particularly to a method for processing a request to update the VC Cache Table. The Virus-free Certificate Cache Table (VCC):

-   -   (701): receives a request to update the VC Cache Table, with a         VC associated with a particular file. Typically, said request         for VC typically comprises:     -   The name, and the type of said file,     -   The size of the file (optionally),     -   The CRC of the file. This is a file signature which identifies         the file.     -   The VC.     -   (702) tests whether or not the received VC is valid.

The received VC is checked to be sure it is valid and has not been corrupted for instance by a malicious system sending corrupted VCs. Typically, the following VC validation is processed:

-   -   The VC is authenticated using its “certificate signature” file,     -   The VC “validity” is correct,     -   The VC has not been revoked.     -   If the received VC is valid:     -   (704) tests whether or not the VC Cache Table is full:     -   If the VC Cache Table is full:     -   (705) deletes some records of the VC Cache Table (706).         Typically, the records which are deleted are selected according         to the “Last_(—)Date” and “Number_(—)Hits” fields. For instance,         the records having the oldest “Last_(—)Date” and the lowest         “Number_(—)Hits” are deleted.     -   (707) updates the VC Cache Table (706), with the received VC.         Either a new record is created in the VC Cache Table, or a new         VC section is created in an existing record.     -   (708) updates one or multiple remote VC Caches. The VCC forwards         the received request to one or multiple VC Caches. The address         of said VC Caches is typically a predefined parameter of the         VCC. This step is obviously bypassed when no remote VC Cache is         defined.     -   If the VC Cache Table is not full:     -   Continues in step (707) to update the VC Cache Table with the         received VC.     -   If the received VC is not valid:     -   (703) stores an information indicating that a request for VC         Cache update has been received with an invalid VC, and discards         the request.         Virus-free Certificate Proxy

The Virus-free Certificate Proxy (VCP) (107) acts as a single Virus-free Certificate Authority (VCA) within the LAN/WAN network in order to provide Virus-free Certificates to any system attached to the LANA/AN network. When multiple VCAs are available (for instance, one VCA per software provider), the VCP identifies a VCA which has authority to build a Virus-free Certificate (VC) for a particular file, and retrieves said Virus-free Certificate (VC) from said identified VCA.

A VC Proxy is a method and system for retrieving a VC for a particular file, from one or multiple VCAs. A VC Proxy processes requests for VCs (a request for VC is a request to retrieve one VC for a particular file). The method of processing each received request for VC comprises the steps of:

-   -   Identifying a VCA which has authority to build a VC for said         particular file,     -   Retrieving said VC from said identified VCA,     -   When said requested VC has been retrieved, answering the         received request for VC, with a positive response comprising         said VC.

Furthermore, the step of identifying a VCA which has authority to build a VC for a particular file, comprises the further steps of:

-   -   Retrieving the identifier of the VCA, using information         comprised within the received request for VC, or     -   Identifying the VCA using     -   information comprised within the received request for VC, and     -   information retrieved from a configured VC Proxy Table.     -   If no VCA is identified:     -   Identifying a Virus-free Certificate Relay (VCR) system, which         can provide the identifier of a VCA.

Retrieving from said identified Virus-free Certificate Relay (VCR) system, the identifier of a VCA which has authority to build said VC.

If no VCR system is identified,

-   -   identifying a configured default VCA using information retrieved         from a configured VC Proxy Table.

Furthermore, the step of identifying a Virus-free Certificate Relay (VCR) system, which can provide the identifier of a VCA which has authority to build said VC, comprises the further steps of:

-   -   Retrieving the identifier of a Virus-free Certificate Relay         (VCR), from information comprised within said received request         for VC, or     -   Identifying a Virus-free Certificate Relay (VCR) system using         information comprised within said received request for VC, and         using information retrieved from a configured VC Proxy Table.

Furthermore, the step of retrieving said VC for said specific file from said identified VCA, consists in either:

-   -   Retrieving the VC, using a short request sent to said identified         VCA and which does not comprise said specific file; or     -   Retrieving the VC, using a full request sent to said identified         VCA and which comprises said specific file.

Typically, the VCP is either a dedicated network device attached to the LAN/WAN network, or an existing network device (for instance an existing WEB Server) which is enhanced to provide the VCP functions.

Virus-free Certificate Proxy Environment

FIG. 8 depicts the logical anti-virus entities involved in a VC Proxy environment. All the systems are attached to the LAN/WAN network (802). The WAN may be for instance the Internet network, or/and a company private Intranet network. One or multiple Virus-free Certificate Authorities (VCAs) (804) are attached to the LAN/WAN network (802). They have authority to build Virus-free Certificates (VCs). Typically, each VCA has authority to build the VCs associated with specific files. There can be:

-   -   one VCA for each software provider (companies such as IBM,         Symantec). For instance, IBM may provide and attach one VCA to         the Internet, in order to build VCs associated with any file         owned by IBM (for instance any software package such as “Host On         Demand”). For instance, the Symantec company may attach one VCA         to the Internet, in order to build a VC associated with any         anti-virus product update (such as a virus signature file).     -   one VCA for each company having authority to build VCs for files         owned by other software providers. Such VCA builds VCs on behalf         of software providers, for any or for specific files owned by         said software providers. For instance, the VeriSign company may         provide and attach one VCA to the Internet in order to build VCs         for any (or specific) file owned by IBM.     -   one VCA internal to the private Intranet network of a software         provider. Such VCA has for instance authority to build the VC         associated with specific or any file transmitted across the         Intranet part of the LAN/WAN network.

In addition to these VCAs (804), one or multiple Default VCAs (803) may be attached to the LAN/WAN network. These Default VCAs have authority to build a VC for any file received by any system attached to the LAN/WAN network. Typically, when a system attached to the LAN/WAN network requires a VC for a particular file, and when no VCA (804) is explicitly identified, the Default VCA is used to build this VC.

Virus-free Certificate Relay

Software providers (for instance small companies) which do not provide their own VCA, may have an agreement with other companies providing VCAs. These software providers have to indicate the identifier of one or multiple VCAs having authority to build a VC associated with one or multiple files they own. This indication is then provided by a system called Virus-free Certificate Relay (VCR) (805).

A Virus-free Certificate Relay (VCR) (805) is therefore a system which provides the identifier of one or multiple VCAs having authority to build a VC for one or multiple files. Typically, a VCR is a File Server (for instance an FTP Server) which originates files owned by a software provider, and which indicates the VCA that can be used to build a VC associated with each file.

For instance, a small software provider may indicate on its VCR (typically a FTP server) that the VCs associated with its software packages can be built by one specific VCA (for instance VeriSign). Any system requiring a VC for one of these software packages can then contact the indicated VCA to retrieve said VC.

Virus-free Certificate Proxy Table

FIG. 9 describes the table used by the Virus-free Certificate Proxy (VCP) (801).

Said table (901) (a flat file in a preferred embodiment) is preferably created by the Network Administrator in charge of the LAN/WAN network. The Virus-free Certificate Proxy Table (901):

-   -   associates each file provider (for instance any software         provider such as IBM), with:     -   the identifier of one or multiple VCAs having authority to build         one or multiple VCs associated with one or multiple files         originated by said file provider.     -   the identifier of one or multiple VCRs capable of providing the         identifier of one or multiple VCAs having authority to build one         or multiple VCs associated with one or multiple files originated         by said file provider.     -   optionally associates one or multiple files, with:     -   the identifier of one or multiple VCAs having authority to build         one or multiple VCs associated with each one of said files.     -   the identifier of one or multiple VCRs capable of providing the         identifier of one or multiple VCAs having authority to build one         or multiple VCs associated with each one of said files.

In the VC Proxy Table, each file provider is identified by its characteristics (903). These characteristics comprise:

-   -   The file provider identifier (typically its name).

Optionally, in the VC Proxy Table, each file (or group of files) is identified by its characteristics (905). These characteristics comprise:

-   -   The file identifier,     -   Optionally, the file name, the file type, the file size, and the         file CRC.

In the VC Proxy Table, each file provider, and optionally each file (or group of files), is associated with one or multiple VCA sections. Each VCA section comprises the following information:

-   -   A VC Authority (VCA) identifier,     -   A VC Relay (VCR) identifier,     -   Optionally, a list of anti-virus programs and levels which are         used by said VCA.

The table comprises a list of records (902). There is one record for each file provider, and optionally one record for each file (or group of files), each record comprising the following information:

-   -   (903) a File Provider section, which comprises the following         information (fields in the record):     -   (904) File_(—)Provider. This is the identifier of the software         provider which orignates one or multiple files. Typically, said         identifier is the company name of the software provider (for         instance IBM Corp.).     -   (905) an optional File Template section, which identifies one or         multiple files having the same characteritics. Said         characteristics comprise the following information (fields in         the record):     -   (906) File_(—)Identity. This is the identifier of one or         multiple files within the file template, which has been         originally created by the software provider when it has created         said one or multiple files. Typically, said identifier is         imbedded within each file by the file provider when the file is         created. Said identifier typically comprises the original file         name, version, and language.     -   (907) File_(—)Name. This is the name of one or multiple files.         The File_(—)Name can be either:     -   an explicit file name which identifies a unique file. For         instance a File_(—)Name with the value “setup1” identifies the         file called “setup1”.     -   a wildcarded file name which identifies multiple files. For         instance, File_(—)Name with the value “setup*” identifies any         file which name starts with “setup” (for instance “setup1”,         “setup_(—)ok”, . . . ).     -   (908) File_(—)Type. This is the type of the file (for instance         “exe”, “doc”, “avi” “html”, . . . ). A File_(—)Type identifies         any file of this type. For instance, a File_(—)Type with the         value “exe” identifies all files which type is “exe” (for         instance “start.exe”, “word.exe”, . . . ).     -   (909) File_(—)Size. This is the size of the file.     -   (910) File_(—)CRC. This is the CRC (a file signature) of the         file.

In most cases, the File_(—)Identity field provides sufficient information to identify in a unique way one file template. File_(—)Name, File_(—)Type, File_(—)Size, and File_(—)CRC are therefore optional in the File Template section.

When File_(—)Identity however, does not identify in a unique way a file template (or when the File_(—)Identity cannot be retrieved for a specific file), the combination of File_(—)Name and File_(—)Type may identify in a unique way a file.

If multiple files have the same name and type (for instance, multiple “setup.exe” files originated from multiple sources), each file may then be differentiated in a unique way by its file size (for instance the size of one “setup.exe” is 2 kilobytes, and the size of another “setup.exe” is 4 kilobytes).

In the unlikely event of multiple files having the same name, type, and size, each file is then differentiated in a unique way by its file CRC which is the file signature.

-   -   (911) a Virus-free Certificate Authority (VCA) section, which         comprises the following information (fields in the record):     -   (912) VCA_(—)Id. This is the identifier of a VC Authority         associated with the File Provider (and optionally with the file         template), and which therefore has authority to build one or         multiple VCs for each file originated by said File Provider.         Typically, this is the IP address of the VCA.     -   (913) VCR_(—)Id. This is the identifier of a VC Relay associated         with the File Provider (and optionally with the file template),         and which therefore can provide the identifier of one or         multiple VCAs having authority to build a VC for one or multiple         files originated by said File Provider. Typically, this is the         IP address of the VCR.     -   (914) AV_(—)Checker_(—)List. This is the list of anti-virus         programs and levels used by the VCA identified by VCA_(—)Id when         it builds VCs. This field is typically empty by default, which         means that said list is unknown.

One record typically comprises one VCA section. However, one record possibly comprises multiple VCA sections, if one File Provider has multiple VCAs (for instance, one VCA per anti-virus program).

The VC Proxy Table comprises one default record, which comprises the identifier (VCA_(—)Identifier) of a Default VCA. Possibly, multiple Default VCAs can be defined.

Internal Logic of the Virus-free Certificate Proxy

FIGS. 10 a and 10 b are flow charts which refer to the internal logic of the Virus-free Certificate Proxy (VCP). The VCP:

-   -   receives a request to retrieve a VC for a particular file.         Typically, said request for VC comprises:     -   One particular file,     -   The name, and the type of this file, A list of anti-virus         programs and levels (referred to as         “RCV_(—)AV_(—)Checker_(—)List”), that must be used to build the         VC. The list may be empty, which means that any anti-virus         program and level can be used.     -   Optionally, a reduced VC which only comprises the “issuer” field         (referred to as “RCV_(—)issuer”), and which identifies the         preferred VCA. Said reduced VC provides the VCP with some         selection criteria which can be used when multiple VCs are         available for the same file (typically, one VC available per         VCA).     -   (1002) tests whether or not the identifier of a VCA that can be         used to retrieve a VC, is comprised within the received request         for VC. Said identifier is then referred to as “VCA_(—)S_(—)Id”.         Typically, said VCA identifier may be imbedded within the         received file (for instance in the organization” section), or         possibly derived from any information retrieved from the         received request.     -   If VCA_(—)S_(—)Id is retrieved from information comprised within         the received request:     -   (1003) retrieves one VC associated with the received file from         the VCA identified by VCA_(—)S_(—)Id, using a short request. The         VCP sends a short request to the VCA identified by         VCA_(—)S_(—)Id, in order to retrieve a VC associated with the         received file. Said short request does not comprise the received         file, in order to minimize the traffic across the LAN/WAN         network. Typically, said short request for VC comprises the         following information:     -   The file name, and file type of the received file.     -   The size of the received file,     -   The CRC of the received file. Typically, the file CRC is         calculated using the received file.

The list of anti-virus programs and levels that must be used by the VCA to build the VC. This list may be empty, which means that any anti-virus program and level can be used.

Typically, this list is the list which was received in the request (“RCV_(—)AV_(—)Checker_(—)List”). Possibly, this list can also be a combination of “RCV_(—)AV_(—)Checker_(—)List” and some configuration information of the VCP (for instance a Network Administrator may want to enforce a specific and updated list of anti-virus programs and levels that must be used to build the VC).

Typically, the VCA_(—)S_(—)Id comprises the address (for instance the IP address) of the VCA. Possibly, when no specific VCA can be identified in a unique way, said short request can be sent (broadcast) to all available VCAs.

-   -   (1004) tests whether or not a VC has been successfully retrieved         from the VCA:     -   If a VC has been successfully retrieved from the VCA:     -   (1005) answers the received request for VC, with a positive         response comprising the VC which has been retrieved. Optionally,         the VC is validated before being sent back in the response. The         VCP then waits for the next request for VC.     -   If a VC has not been successfully retrieved from the VCA:     -   (1006) tests whether or not the VCA requires the received file         to build the VC. This test uses information retrieved from the         message which has been received from the VCA in response to the         short request sent in (1003). Said information indicates whether         or not the VCA requires the file to build the VC.     -   If the VCA requires the file to build the VC:     -   (1007) retrieves a VC associated with the received file from the         VCA identified by VCA_(—)S_(—)Id, using a full request.

The VCP sends a full request to the VCA identified by VCA_(—)S_(—)Id, in order to retrieve a VC associated with the received file. This full request comprises the received file.

-   -   (1008) tests whether or not a VC has been successfully retrieved         from the VCA:

If a VC has been successfully retrieved from the VCA:

-   -   (1005) answers the received request for VC, with a positive         response comprising the VC which has been retrieved. Optionally,         the VC is validated before it is sent back in the response. The         VCP then waits for the next request for VC.

If a VC has not been successfully retrieved from the VCA:

-   -   (1009) tests whether or not the VCA is a Default VCA, and when         the VCA is a Default VCA, whether or not another Default VCA is         available. This test preferably uses an indication which is         stored by the VCP when it identifies the VCA, and which is         determined using the VCP Table (1020).     -   If the VCA is a Default VCA, and no other Default VCA is         available:     -   (1010) stores an information indicating that the VCP has not         been able to retrieve a VC associated with the particular file         comprised within the received request for VC. Possibly, a         notification is also sent to a Network Administrator.     -   (1011) answers the received request for VC, with a negative         response which indicates that no VC has been retrieved. The VCP         then waits for the next request for VC.     -   If the VCA is not a Default VCA, or if the VCA is a Default VCA         but another Default VCA is available:     -   (1012) excludes the VCA identified by VCA_(—)S_(—)Id, from the         list of VCAs which can possibly be used to retrieve a VC for the         received request. Goes back to the identification of a VCA, in         step (1002), since another VCA (typically a Default VCA) may be         able to build the VC.     -   If the VCA does not requires the file to build the VC:

The VCA is therefore not able to build the VC.

-   -   Continues in step (1009) to test if the VCA is a Default VCA.     -   If VCA_(—)S_(—)Id is not retrieved from information comprised         within the received request:     -   (1013) tests whether or not the identifier of a VC Relay (VCR)         is comprised within the received request for VC. Said identifier         is then referred to as “VCR_(—)S_(—)Id”. Typically, said VCR         identifier may be imbedded within the received file (for         instance in the organization” section), or possibly derived from         any information retrieved from the received request.     -   If VCR_(—)S_(—)Id is retrieved from information comprised within         the received request:     -   (1014) retrieves the identifier (VCA_(—)S_(—)Id) of a VCA which         has authority to build a VC for the received file, from the VCR         identified by VCR_(—)S_(—)Id. The VCP sends a short request to         the VCR identified by VCR_(—)S_(—)Id, in order to retrieve the         identifier (VCA_(—)S_(—)Id) of a VCA which has authority to         build a VC for the received file. Typically, said short request         comprises the following information:     -   The file name, and file type of the received file.

The size of the received file,

-   -   The CRC of the received file. Typically, the file CRC is         calculated using the received file.     -   The list of anti-virus programs and levels that must be used by         the VCA to build the VC. Possibly, said list is empty, which         means that any anti-virus program and level can be used.         Typically, said list is the list which was received in the         request (“RCV_(—)AV_(—)Checker_(—)List”). Possibly, said list         can also be a combination of “RCV_(—)AV_(—)Checker_(—)List” and         some configuration information of the VCP (for instance a         Network Administrator may want to enforce a specific and updated         list of anti-virus programs and levels that must be used to         build the VC). Typically, the VCR_(—)S_(—)Id comprises the         address (for instance the IP address) of the VCR.     -   (1015) tests whether or not the identifier (VCA_(—)S_(—)Id) of a         VCA has been retrieved from the VCR.     -   If the identifier (VCA_(—)S_(—)Id) of a VCA has been retrieved         from the VCR:     -   Continues in step (1003) in order to retrieve a VC for the         received file, from said identified VCA.     -   If the identifier (VCA_(—)S_(—)Id) of a VCA has not been         retrieved from the VCR:     -   (1016) retrieves the identifier (VCA_(—)S_(—)Id) of a Default         VCA, from the VCP Table (1020).     -   Continues in step (1003) in order to retrieve a VC for the         received file, from said identified VCA.     -   If VCR_(—)S_(—)Id cannot be retrieved from information comprised         within the received request:     -   (1017) retrieves one record (called “Record_(—)S”) from the VC         Proxy Table. Said record is preferably identified by:     -   Its “File_(—)Provider” field, which must be equal to (or must         comprise) either:     -   the “RCV_(—)issuer” which has been received in the request for         VC, or     -   the file provider identifier which is possibly imbedded in the         file.

Possibly, said record can also be identified by:

-   -   Its “File_(—)Identity” field, which must be equal (or must         comprise) the file identifer retrieved from the file.     -   Optionally, its “File_(—)Name”, “File_(—)Type” “File_(—)Size”         and “File_(—)CRC” which must be equal to respectively) the name,         type, size, and CRC of received file.

At least a Default record can be identified.

-   -   (1018) tests whether or not the identifier of a VCA that can be         retrieved from the VCP Table. A VCA section is primarily         selected within “Record_(—)S”. Preferably, the selected VCA         section is a section which “AV_(—)Checker_(—)List” is equal to         (or comprises) “RCV_(—)AV_(—)Checker_(—)List”. The identifier of         a VCA can be retrieved from the VCP Table, when said selected         VCA section comprises a VCA Identifier in its “VCA_(—)Id” field.     -   If the identifier (called VCA_(—)S_(—)Id) of a VCA can be         retrieved from the VCP Table:     -   Continues in step (1003) in order to retrieve a VC for the         received file, from said identified VCA.

If the identifier of a VCA cannot not be retrieved from the VCP Table:

-   -   (1019) tests whether or not the identifier of a VCR that can be         retrieved from the VCP Table. A VCA section is primarely         selected within “Record_(—)S”. Preferably, the selected VCA         section is a section which “AV_(—)Checker_(—)List” is equal to         (or contains) “RCV_(—)AV_(—)Checker_(—)List”. The identifier of         a VCR can be retrieved from the VCP Table, when said selected         VCA section comprises a VCR Identifier in its “VCR_(—)Id” field.     -   If the identifier (called VCR_(—)S_(—)Id) of a VCR can be         retrieved from the VCP Table:     -   Continues in step (1014) in order to retrieve, from the VCR         identified by VCR_(—)S_(—)Id, the identifier of a VCA which has         authority to build a VC for the received file.     -   If the identifier of a VCR cannot be retrieved from the VCP         Table:     -   Continues in step (1016) in order to retrieve the identifier         (VCA_(—)S_(—)Id) of a Default VCA, from the VCP Table (1020).         Advantages     -   The VCP off-loads the system requesting a VC, of the         identification of the VCA which has to build the VC.     -   The VCP reduces the load of VCAs, because it identifies the         particular VCA which has to build a particular VC and can spread         the requests for VCs across multiple VCAs.     -   The VCP enables the use of multiple VCAs within the network. For         instance, multiple VCAs can share the load of building VCs, and         can possibly use different anti- virus checkers.

While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood that various changes in form and detail may be made therein without departing from the spirit, and scope of the invention. 

1. A method for use in a virus-free certificate proxy, of retrieving from one or a plurality of virus-free certificate authorities a virus-free certificate certifying that a file is virus-free, said method comprising the steps of: receiving a virus-free certificate request for a file; selecting a virus-free certificate authority having authority to generate a virus-free certificate for the file; requesting the virus-free certificate to the selected virus-free certificate authority; receiving from the selected virus-free certificate authority the generated virus-free certificate; and sending back in response to the virus-free certificate request the received virus-free certificate; and wherein the virus-free certificate request comprises: a list of one or a plurality of anti-virus programs to execute on the file to determine whether the file is virus-free or not and the file for which the virus-free certificate is requested.
 2. The method according to claim 1 wherein the virus-free certificate request comprises: an identification of the file comprising: a file identity, or an identification of the provider of the file, or both.
 3. The method according to claim 1 wherein the step of selecting a virus-free certificate authority having authority to generate a virus-free certificate for the file comprises the further steps of: determining whether the virus-certificate request comprises an identification of a virus-free certificate authority or not; and if the virus-free certificate request does not comprises an identification of a virus-free certificate authority, selecting a virus-free certificate authority referring to a first table, said first table comprising for each identified file a list of one or a plurality anti-virus programs, each list being associated with a virus-free certificate authority.
 4. The method according to claim 3 wherein the step of selecting a virus-free certificate authority referring to a first table comprises the steps of: determining whether the file is identified or not in the first table; if the file is identified in the first table, selecting a list of one or a plurality of anti-virus programs according to a list of one or plurality of anti-virus programs that is included in the virus-free certificate request; determining whether a virus-free certificate authority is associated with the selected list or not; and if a virus-free certificate authority is associated with the selected list, selecting the virus-free certificate authority associated with the selected list.
 5. The method according to claim 3 wherein the step of selecting a virus-free certificate authority referring to a first table comprises the steps of: determining whether the file is identified or not in the first table; if the file is identified in the first table, selecting a list of one or a plurality of anti-virus programs according to a list of one or plurality of anti-virus programs that is included in the virus-free certificate request; determining whether a virus-free certificate authority is associated with the selected list or not; if a virus-free certificate authority is not associated with the selected list, determining whether a virus-free certificate relay is associated with the selected list or not; and if a virus-free certificate relay is associated with the selected list, selecting a virus-free certificate authority referring to the virus-free certificate relay.
 6. The method according to claim 5 wherein the step of selecting a virus-free certificate authority referring to a virus-free certificate relay comprises the step of: selecting a virus-free certificate authority referring to a second table in said virus-free certificate relay, said second table comprising for each identified file a list of one or a plurality of anti-virus programs, each list being associated with a virus-free certificate authority.
 7. The method according to claim 3 wherein the step of selecting a virus-free certificate authority referring to a first table comprises the steps of: determining whether the file is identified or not in the first table; and if the file is not identified in the first table, selecting a default virus-free certificate authority in the first table.
 8. The method according to claim 1 wherein the step of selecting a virus-free certificate authority having authority to generate a virus-free certificate for the file comprises the further steps of: determining whether the virus-certificate request comprises an identification of a virus-free certificate authority or not; if the virus-free certificate request does not comprise an identification of a virus-free certificate authority, determining whether the virus-certificate request comprises an identification of a virus-free certificate relay or not; and if the virus-certificate request does not comprise an identification of a virus-free certificate relay, selecting a virus-free certificate authority referring to a first table, said first table comprising for each identified file a list of one or a plurality anti-virus programs, each list being associated with a virus-free certificate authority, or a virus-free certificate relay or both.
 9. The method according to claim 1 wherein the step of selecting a virus-free certificate authority having authority to generate a virus-free certificate for the file comprises the further steps of: determining whether the virus-certificate request comprises an identification of a virus-free certificate authority or not; and if the virus-free certificate request comprises an identification of a virus-free certificate authority, selecting the virus-free certificate authority identified in the virus-free certificate request.
 10. The method according to claim 1 wherein the step of selecting a virus-free certificate authority having authority to generate a virus-free certificate for the file comprises the further steps of: determining whether the virus-certificate request comprises an identification of a virus-free certificate authority or not; if the virus-free certificate request does not comprises an identification of a virus-free certificate authority, determining whether the virus-certificate request comprises an identification of a virus-free certificate relay or not; and if the virus-certificate request comprises an identification of a virus-free certificate relay, selecting a virus-free certificate authority referring to the virus-free certificate relay.
 11. The method according to claim 1 wherein the generated virus-free certificate comprises: a file signature for certifying that the file is declared virus-free by the selected virus-free certificate authority.
 12. The method according to claim 1 wherein the virus-free certificate further comprises: a file identification; a virus-free certificate authority identification; a public key for decrypting the file signature; a certificate signature for authenticating the virus-free certificate; and an indication of the virus-free certificate validity.
 13. A system implementing a virus-free certificate proxy, said system comprising a processor that executes a program for implementing a method having the steps of: receiving virus-free certificate request for a file; selecting a virus-free certificate authority having authority to generate a virus-free certificate for the file; requesting the virus-free certificate to the selected virus-free certificate authority; receiving from the selected virus-free certificate authority the generated virus-free certificate; and sending back in response to the virus-free certificate request the received virus-free certificate; and wherein the virus-free certificate request comprises: a list of one or a plurality of anti-virus programs to execute on the file to determine whether the file is virus-free or not and the file for which the virus-free certificate is requested.
 14. A computer program recorded on a computer-readable medium and comprising instructions for executing a method having the steps of: receiving virus-free certificate request for a file; selecting a virus-free certificate authority having authority to generate a virus-free certificate for the file; requesting the virus-free certificate to the selected virus-free certificate authority; receiving from the selected virus-free certificate authority the generated virus-free certificate; and sending back in response to the virus-free certificate request the received virus-free certificate; and wherein the virus-free certificate request comprises: a list of one or a plurality of anti-virus programs to execute on the file to determine whether the file is virus-free or not and the file for which the virus-free certificate is requested.
 15. A method for use in a virus-free certificate proxy, of retrieving from one or a plurality of virus-free certificate authorities a virus-free certificate certifying that a file is virus-free, said method comprising the steps of: receiving a virus-free certificate request for a file; selecting a virus-free certificate authority having authority to generate a virus-free certificate for the file; requesting the virus-free certificate to the selected virus-free certificate authority; receiving from the selected virus-free certificate authority the generated virus-free certificate; and sending back in response to the virus-free certificate request the received virus-free certificate; determining whether the virus-certificate request comprises an identification of a virus-free certificate authority or not; and if the virus-free certificate request does not comprises an identification of a virus-free certificate authority, selecting a virus-free certificate authority referring to a first table, said first table comprising for each identified file a list of one or a plurality anti-virus programs, each list being associated with a virus-free certificate authority, and wherein the virus-free certificate request comprises: a list of one or a plurality of anti-virus programs to execute on the file to determine whether the file is virus-free or not, and an identification of the file comprising: a file identity, or an identification of the provider of the file, or both. 